Loss of Sovereignty

According to the Financial Times, Merkel wants to make the new bail-out conditional on Greece giving up fiscal sovereignty to a “budget commissioner” with veto power who is to be appointed by the eurozone finance ministers. Without knowing the precise details, of course, it is too soon to evaluate this possible proposal, however, one is left wondering, first, if this is really necessary and, second, if it is constitutional. Besides, the issue at hand is that the Greek government cannot implement reforms fast enough because of resistance by special interest groups and unions. How is a budget commissioner going to help with reforms outside the public sector?